This invention is generally directed to processes for the preparation of liquid and dry toners, and more specifically to processes employing a supercritical fluid, such as carbon dioxide, for the preparation of developer compositions containing small polymeric particles, for example, in embodiments, with an average diameter of from about about 0.1 micron to about 5 microns. More specifically, the present invention is directed to economic processes for the preparation of micron and submicron size polymeric particles, useful as liquid and dry electrophotographic developer compositions which processes comprise, in embodiments, forming a melt mixture comprised of a polymer resin or resins, a colorant, a charge director additive, and a hydrocarbon liquid carrier, to obtain a first suspension of colored polymeric particles with an area average diameter of from about 2 to about 100 microns; dispersing said first suspension in a supercritical fluid medium and thereafter continuously feeding the resultant dispersion to a liquid fluidizing means at a pressure of from about 800 to about 4,000 pounds per square inch to obtain a second suspension comprising supercritical fluid and liquid developer mixture containing colored polymeric particles with an area average diameter, in embodiments, of less than about about 2.0 microns and a solids content of about 10 to about 90 weight percent; and reducing the pressure to ambient levels to evaporate, and optionally recovering the supercritical fluid medium from the second suspension, wherein there results a liquid developer mixture containing colored polymeric particles with an area average diameter of less than about about 2.0 microns and a solids content of about 10 to about 90 weight percent is obtained, for example. As indicated herein, the finely divided polymer particles obtained with the process of the present invention can, for example, be selected as liquid and dry electrophotographic developer compositions.
The formation of small polymeric particles for use in liquid and dry electrophotographic developer compositions by particle size reduction or comminution of larger particles has been generally accomplished by, for example, milling or grinding processes for extended periods of time wherein polymer particles suspended in a non-dissolving liquid are milled with optional heating to form particles having reduced particle size properties. With these processes, it has been difficult to achieve low cost, clean, that is for example with no, or substantially no, impurities from the milling media or apparatus on the surface of the resulting particles, and/or dry particles of small particle size. The particles formed by milling or grinding processes are generally larger than 2.0 micrometers thus they are not suitable as liquid and dry electrophotographic developer compositions, particularly for high quality color printing applications unless lengthy attrition times, generally exceeding 6 hours, are used to obtain particles on the order of 2 microns volume average diameter. Thus grinding or attrition, especially fluid energy milling, of large particles to the size needed for liquid and dry developer compositions, that is for example from about 0.1 to about 5 microns volume average diameter, is often not desirable both from an economic and functional viewpoint. Further, processes such as spray drying of polymers suspended in solvent can result in polymer particles with particle sizes much larger than about one micron and possessing a broad size distribution range including fibers and strands of filamented resins, as well as trapping of solvent which interferes with the viability of the particles as developers. Moreover, solvent recovery in these processes is considered very costly.
Trout et al, in U.S. Pat. No. 4,783,389, issued Nov. 8, 1988 disclose a process for the preparation of toner particles for liquid electrostatic imaging comprising: (a) mixing a thermoplastic resin and a nonpolar liquid at a temperature sufficient to plasticize and liquify the resin and below that at which the non-polar liquid boils and the resin decomposes; (b) cooling the mixture to form resin particles in the nonpolar liquid; and (c) reducing the size of the resin particles to below about 30 micrometers by passing the product of step (b) through at least one liquid jet interaction chamber at a liquid pressure of at least 1,000 psi (68 Bars), for example, using a MICRCOFLUIDIZER.RTM. from Microfluidics Corporation. The process produces liquid electrostatic developer useful in copying, making proofs, including digital proofs, and the like. The MICROFLUIDIZER.RTM. method suffers from several disadvantages including frequent and recurring jet nozzle clogging with particles greater than 50 microns in diameter. Moreover, resin filaments and large particles are formed at operating pressures of greater than about 500 Bars. Thus, at typical Microfluidizer.RTM. processing pressures recommended by Trout et al, polymer suspensions in nonaqueous solvents tend to destabilize and lead to agglomerated particles that are not suitable for liquid or dry electrophotographic developers.
Komuro et al, in U.S. Pat. No. 5,123,962, issued Jun. 23, 1992, disclose a suspension comprising a dispersing medium containing at least 2% by weight of a fine particle cellulose material having a 50% cumulative volume diameter of from 0.3 to 6.0 micrometers. The suspension is obtained by a process comprising subjecting a cellulosic material to a depolymerization pretreatment, followed by wet grinding in a container containing a grinding medium and equipped with a rotary blade for forced stirring of the medium. The suspension has excellent viscosity, water retention properties, stability, and palatability.
El-Sayed et al, in U.S. Pat. No. 5,053,306, issued Oct. 1, 1991, disclose a process for the preparation of toner particles for electrostatic liquid developers comprising: (a) dispersing at ambient temperatures a colorant, an A-B diblock copolymer grinding aid, and a carrier liquid; (b) adding to the dispersion a thermoplastic resin and dispersing at an elevated temperature to plasticize and liquify the resin; (c) cooling the dispersion while grinding with particulate media; (d) separating a dispersion of toner particles having an average by area particle size less than 10 micrometers, from the particulate grinding media; and (e) adding during or subsequent to step (b) at least one ionic or zwitterionic charge director compound. Steps (a) and (b) can be combined by adding the thermoplastic resin to the other ingredients and dispersing at an elevated temperature. The liquid developers are useful in,copying, in making color proofs, and the like.
Wasmund et al, in U.S. Pat. No. 5,168,022, issued Dec. 1, 1992, disclose a process for preparing a photoconductive pigment having a small particle size, a polymorph of a pigment is produced by a conversion process wherein a seed amount of the desired polymorph of the pigment and a larger amount of another polymorph of the pigment are subjected to a liquid jet interaction process.
Wong et al, in U.S. Pat. No. 4,960,667, issued Oct. 2, 1992 disclose a positively charged liquid developer composition comprised of resin particles, a hydrocarbon, laked carbon black particles, and a charge director wherein the composition is prepared in a shot mill attritor with steel balls.
Chan et al, in U.S. Pat. No. 4,917,986, issued Apr. 17, 1990, disclose a positive, liquid electrostatic developer consisting essentially of (a) a nonpolar liquid having a Kauri-butanol value of less than 30, present in a major amount, (b) thermoplastic resin particles having dispersed therein a phosphorous containing compound defined therein which is substantially insoluble or immiscible in the nonpolar liquid at ambient temperatures, the resin particles having an average by area particle size of less than 10 microns, and (c) a nonpolar liquid soluble ionic or zwitterionic charge director compound, and a process for preparation. The preparation process comprises (a) dispersing the resin, the phosphorous compound at elevated temperature, (b) cooling with or without stirring or while grinding, (c) separating the dispersion of toner particles from the particulate media, and (d) adding to the dispersion during or subsequent to step (a) a nonpolar liquid soluble ionic or zwitterionic charge director compound.
The use of supercritical fluids (SCF) in materials and chemical processing systems is known, for example, for forming homogeneous polymer blends in U.S. Pat. No. 5,290,827; depositing thin films in U.S. Pat. No. 4,737,384; organic synthetic reaction solvent media in U.S. Pat. No. 5,241,048; polycarbonate polymer purification in U.S. Pat. No. 4,918,160; and enhanced liquid extraction in U.S. Pat. No. 4,547,292.
Other references of interest include: U.S. Pat. Nos. 4,476,210 and 4,816,370 to Croucher et al., which disclose liquid developers and methods for making; and U.S. Pat. No. 5,306,590 to Fendler which discloses high solids liquid developer concentrates.
The aforementioned Trout et al., U.S. Pat. No. 4,783,389, which utilizes a MICROFLUIDIZER.RTM. device to achieve particle size reduction relies upon two principle mechanisms: particle-particle collisions between opposing liquid streams and cavitation. Using a MICROFLUIDIZER.RTM. device in a conventional manner for the preparation of liquid dispersions of very fine particles as described by Trout et al., has several inherent complications and operational limitations, including, for example: 1) a requirement that the feed solution to be fluidized be hot, at a temperature of about 80 to about 100.degree. C., and the initial particle size be less than about 50 micrometers; 2) the MICROFLUIDIZER.RTM. device is energy intensive requiring an air compressor to attain supersonic high pressures; 3) the device is operationally man power intensive in that it has various valving and orifices which can potentially readily clog and require regular dissembly and tedious cleaning thereby limiting potential for continuous operation; and 4) the device produces liquid ink developer formulations that tend to be unstable and have limited storage shelf-life in that the formulations may undergo catastrophic formulation failure on standing at room temperature as manifested by a congealing of the suspended resin particles into large monolithic solid masses which are difficult or nearly impossible to redisperse without resorting to high energy means. Moreover, resin filaments and large particles are formed at operating pressures greater than 500 Bars in the absence of a supercritical fluid, which pressures are typical of MICROFLUIDIZER.RTM. processing/operating pressures.
Use of the aforementioned shot mill attritor technique for achieving resin in hydrocarbon formulation dispersion and particle size reduction of less than about 10 microns average diameter as, for example, in Wong U.S. Pat. No. 4,960,667, typically a very energy and time intensive process and noisy unit operation, results in metal contamination from the steel balls which may require an additional magnetic filtration step. The shot mill has a rather limited operational void volume where the formula is processed, even for very large attritors, thereby prohibiting rapid and continuous large scale production.
There thus remains a need for an economic and convenient process of obtaining very small polymeric particles, and more specifically micron and submicron polymeric particles, without the complications and disadvantages of the aforementioned prior art devices and processes. Further, there is a need for particle size reduction or comminution processes for obtaining clean, optionally dry and small polymeric particles, for example, from about 0.1 to about 5 microns and preferably less than about 2.0 microns in volume average diameter as determined by a scanning electron microscope or a Horiba Capa-700 particle size distribution analyzer. Still further, there is a need for heterogenous or non-dissolving particle size reduction processes that permit low cost, clean, and optionally dry, or nonaqueous, micron and submicron polymeric particles that can be selected as liquid and dry electrophotographic developer compositions, carrier powder coatings, photoconductor pigment-resin coating suspensions, and as toner additives for enhanced photoreceptor cleaning. Another need is the ability to directly produce high solids resin particle suspensions for use as liquid developers and the like liquid formulations without the need for an intermediate concentration step.